1. Field of the Invention
This invention relates to a billet control method in a horizontal continuous casting system. More particularly it relates to a method for controlling the length of pushing back of the billet towards the mold with the aid of the pinch rolls during the top cycle of the intermittent billet extraction process in order to promote stable growth of the newly formed billet shell.
2. Description of the Prior Art
In a conventional horizontal continuous casting system, billet extraction is by an intermittent process according to which the billet is drawn out of the mold at preset speed a preset length and halted for some time so as to allow for stable growth of the newly formed shell, the billet being again extracted after termination the growth of the shell. During the stop cycle, the billet is contracted in size, thus possibly causing rupture of the newly formed shell. In the conventional process, the following methods (a) or (b) are used so as not to obstruct free billet contraction between the pinch rolls and the mold. (a) The method of opening the pinch rolls apart during the stop cycle, according to which the pinch rolls are opened apart during the stop cycle to permit free contraction of the extracted billet between the pinch rolls and the mold so as to prevent cracking or rupture of the newly formed shell.
In the method (a), the previously formed shell and the newly formed shell are affixed to each other only slowly so that it is not possible to elevate the speed of the extraction cycle. In addition, the large-size billet tends to be ruptured on account of the higher frictional resistance between the billet and the rolls. Also, the extraction resistance in the mold is markedly changed with the mold profile, the temperature of the molten steel or the steel type, resulting in breakouts and obstruction of stable casting due to obstruction of free contraction of the billet and rupture of the newly formed shell. (b) The method of applying a reset pressure to the billet forcing back side of the driving hydraulic motor during the billet forcing back step. According to this method the billet forcing back step is provided in continuation to the stop cycle of the intermittent extraction process. During the billet forcing back step, a preset pressure is applied to the billet forcing back side of the pinch rolles driving hydraulic motor for pushing the billet back towards the mold for positively promoting billet contraction so as to prevent rupture of the newly formed shell. With the present method (b), however, the billet is forced back with a constant preset force despite fluctuations in the roll and/or mold resistances. The result is that occasionally the forcing back pressure is insufficient thus causing billet breakouts due to shell rupture, or the forcing back pressure becomes too strong thus again causing billet breakouts due to buckling of the newly formed shell.
With the above described conventional methods (a), (b) consisting in opening the pinch rolls apart or applying the preset pressure to the billet forcing back side of the driving hydraulic motor, it is not possible to successfully deal with fluctuations in the resistance inside the mold or the roll resistance, thus causing rupture or buckling of the newly formed shell and resulting breakouts.